This invention relates to the stabilization of certain polymers against the degradative effects of heat. This invention further relates to improving the heat stability imparted to these polymers by a known class of diorganotin compounds.
It is well known that many diorganotin compounds are effective heat stabilizers for vinyl chloride polymers and acrylonitrile-butadiene-styrene terpolymers. Considered as a class, compounds containing tin-sulfur bonds, such as dibutyltin-S,S'-bis(isooctyl mercaptoacetate), retard heat-induced degradation and discoloration over a longer period of time than diorganotin compounds with tin-oxygen bonds. The presence of sulfur-containing organotin compounds may be undesirable in those instances where a completely colorless formulation is required, since these compounds may impart a yellow tinge to the formulation. In addition, many of these stabilizers have unpleasant odors at the elevated temperatures employed to process the polymer formulation. Formulations containing diorganotin derivatives of carboxylic acids, such as dibutyltin dilaurate and dibutyltin maleate, are often superior with regard to initial color to those incorporating sulfur-containing stabilizers, but may lack the desired long term heat stability.
Diorganotin maleates retard initial discoloration and also impart relatively long term heat stability, however at elevated temperatures these compounds produce vapors which not only have an unpleasant odor, but are particularly irritating to the eyes and respiratory tract. Many of the diorganotin carboxylates decompose to some extent at elevated temperatures yielding vapors of the corresponding acids. While not as noxious as the heated diorganotin maleates, these vapors are unpleasant. The diorganotin carboxylates should therefore be employed only when adequate ventilation is available. Diorganotin carboxylates are conventionally prepared by reacting the acid with a stoichiometric amount of a diorganotin oxide. One mole of oxide is present for every 2 moles of carboxyl (--COOH) groups. It has been found that vaporization of diorganotin carboxylates can be reduced if these compounds are prepared using a stoichiometric excess of the corresponding diorganotin oxide. Preferably between 1.1 and about 4 moles of oxide are reacted for every two moles of carboxyl groups. The odor of molten polymer formulations containing these "overbased" diorganotin carboxylates, particularly the maleates, may still be so offensive and irritating as to preclude the use of these stabilizers for many applications, irrespective of the high level of heat stability that they impart to a polymer formulation. It is therefore an objective of this invention to eliminate or reduce to a negligible extent the vaporization of overbased diorganotin carboxylates that occurs at temperatures between 150.degree. and 200.degree. C., the range wherein vinyl chloride polymers and acrylonitrile-butadiene-styrene terpolymers are conveniently processed. It has now been found that this objective can be achieved by combining an "overbased" diorganotin carboxylate with a mono- or dicarboxylic acid containing from 8 to 20 carbon atoms.
Surprisingly the presence of a free carboxylic acid or anhydride not only reduces odor, but also significantly improves the heat stability imparted to polymer formulations by overbased diorganotin carboxylates. The latter is quite unexpected, since free carboxylic acids and anhydrides are not effective heat stabilizers.